OBJECTIVE: Increased BMI is a robust risk factor for type 2 diabetes. Paradoxically, South Asians have relatively low BMIs despite their high prevalence of type 2 diabetes. We examined the association between BMI and incident type 2 diabetes because detailed prospective cohort data on this topic in Asians are scarce. RESEARCH DESIGN AND METHODS: This study was a prospective analysis of 37,091 men and women aged 45-74 years in the Singapore Chinese Health Study, using Cox regression analysis. RESULTS: Risk of incident type 2 diabetes significantly increased beginning with BMIs 18.5-23.0 kg/m(2)(relative risk 2.47 [95% CI 1.75-3.48]) and continued in a monotonic fashion across the spectrum of BMI. Results were stronger for younger than for older adults. CONCLUSIONS: BMIs considered lean and normal in Singaporean Chinese are strongly associated with increased risk of incident type 2 diabetes. This association weakened with advanced age but remained significant.